In a motor drive controller for driving a motor without using a sensor device for detecting a position of a rotor (hereinafter referred to as a “position sensor-less system”), there is a need to perform positioning of the rotor at the time of starting. As a method of performing the positioning, for example, a lock energization method for locking a rotor at a predetermined position by switching on upper and lower switching elements, each of which has a predetermined phase and is provided in an inverter circuit for supplying power to coils of a motor, for a fixed time is known.
An example of a drive controller for a motor is described in JP-A-2011-182505 in which a rotor is controlled such that a lock current is increased from the onset of starting over a predetermined time until a magnitude of the lock current reaches a predetermined value, then an energized state of the lock current having a predetermined magnitude continues for a fixed time, and thereby the rotor is reliably stopped at a predetermined position.
Meanwhile, when the motor for rotating, for instance, a driven element having large moment of inertia is allowed to be driven by the motor drive controller, a starting time required to perform lock energization at the time of starting to enable the rotation of the motor may be prolonged.
For example, in a case in which a driven element having large inertia is mounted on a rotor like a fan motor using a large vane (an impeller), when the vane moves toward a lock position by performing lock energization, the rotor is rotated by inertia of the vane up to a position at which the vane exceeds the lock position. As a result, torque is applied to the rotor in the opposite direction to return to the lock position, and the rotor is stopped at the position beyond the lock position for the moment, and then moves toward the lock position in a direction opposite to the just previous direction. A position at which a rotational speed of the rotor becomes zero by repeating this operation approaches the lock position, and the rotor can be finally stopped at the lock position. That is, when the moment of inertia of the rotor is large, an ON duty PWM signal having a magnitude to some extent needs to be output to the motor to move the rotor toward the lock position. However, although the ON duty PWM signal is output for a fixed time, the rotor cannot be completely locked immediately. As the inertia of the vane increases, a time until the rotor comes to a stop is prolonged.